1. Field of the Invention
This invention relates generally to apparatus which permits disreefing of line-reefed parachutes in a clustered parachute system and, more particularly, to apparatus for simultaneous disreefing of all parachutes in a cluster.
2. History of the Prior Art
The early stages of deployment of any parachute or cluster of parachutes involve unsteady aerodynamics, and consequent unsteady loading of the lines between the parachute and the load. A principal advantage is the faster inflation rate for a cluster of smaller parachutes, since the "apparent mass" of a cluster of smaller parachutes is smaller than the "apparent mass" of an equivalent large single parachute. Therefore, a clustered parachute system, with rapid inflation of each parachute in the cluster, is preferable for applications in which large drag areas must become effective very rapidly after deployment. It is very important, however, to obtain simultaneous inflation of each parachute in the cluster and even distribution among all the parachutes in the cluster of the load-supporting drag force produced by the entire clustered parachute system. Reefing is a means by which the parachutes, when initially deployed, are limited in the extent to which they can open until a certain degree of inflation stability and load control have been obtained. Reefing is provided by passing a frangible line through rings attached to the periphery of each parachute. The reefing line has a length less than that of the periphery in its fully deployed state so that the parachutes fill up with air after deployment but are not free to open fully to exert their maximum drag force on the load. After stability is attained by the line-reefed parachutes, their reefing lines are cut to permit the parachutes to open up further and exert even larger drag forces on the load suspended below.
If the reefing lines are not all cut at the same time, the parachutes will not inflate simultaneously. This will allow one or more of the parachutes to inflate earlier, generating more than an appropriate share of the total drag force applied to the load. To avoid catastrophic failure of the lines or of the fabric of any parachute that inflates too early, it therefore becomes necessary to overdesign each of the parachutes. Since such overloads last for relatively short times, such overdesigning is expensive and adds undesirable weight to the system as a whole. Furthermore, those parachutes which lag in the inflation process generally will not inflate until several seconds later which causes the drag area and hence the drag force produced early in the deployment process to be lower than expected so that the payload does not decelerate as desired.
A clustered parachute system typically is deployed from a deployment bag attached to the falling load. Upon deployment, the cluster of parachutes is presented to the air stream in a random manner, causing corresponding random inflation of the individual parachutes. Even if reefing of the parachutes is not required to control the development of drag force of each parachute, short time delay reefing can be used to initialize the opening of the skirt of each canopy in a symmetric manner.
As noted earlier, at an appropriate time, each parachute is disreefed, generally by a cutting element severing one or more lines upon command. It is common practice to provide individual pyrotechnic delays to reefing line cutters which are actuated when the load lines connecting the parachutes to the payload reach their full stretch. Such pyrotechnic time delay systems have a 20-30% tolerance in the time delay provided, causing nonsimultaneous inflation of the parachutes of the cluster.
One solution, offered in U.S. Pat. No. 3,780,970 to Pinnell, teaches the use of a generally triangularly-shaped web, located centrally of the parachutes in the cluster, to provide control of the parachute skirts during the reefing stage. Some of the parachute reefing rings of each parachute are held together by the web assembly to limit the opening of the parachute skirt. Since part of each parachute periphery is reefed while the rest of the parachute is allowed to inflate, this causes asymmetric loading. Where high performance is needed, generally in highly loaded parachute systems, such asymmetric loading will cause failure of individual parachutes and thus of the entire system. In addition, the Pinnell web assembly must be strong (and hence heavy).
There is, therefore, a need for apparatus that is light and simple and that enables each chute in the clustered parachute system to have symmetrically applied loading and also enables the simultaneous disreefing of a plurality of parachutes forming a clustered parachute system. Such a system should also be capable of effecting disreefing in stages, so as to avoid overloading of individual parachutes and permitting greater control on the rate of descent of a parachuted load.